The present invention relates to a method for producing magnetic recording medium and an apparatus for producing a magnetic recording medium.
More particularly, the invention relates to a method for producing a magnetic recording medium of the iron nitride thin film type and an apparatus for producing such a recording medium.
A widely used conventional magnetic recording medium is manufactured by applying a magnetic coating in which ferromagnetic powder is dispersed in an organic binder onto a non-magnetic substrate and dried thereon. This coating-type magnetic recording medium, however, has the disadvantages that the saturation magnetization is small because metal oxide powder is mainly used as the ferromagnetic powder, that the concentration of the ferromagnetic material in the magnetic layer cannot be made high due to the presence of the organic binder making the coating-type magnetic recording medium unsuitable for high density recording, and that the manufacturing process for this type of recording medium is complicated.
Recently in response to an increased demand for high density recording, a magnetic recording medium has been developed in which a metal thin film is formed on a nonmagnetic substrate. To produce this magnetic recording medium, a metal thin film is formed on a nonmagnetic substrate by a vapor deposition method such as vacuum deposition, sputtering, or ion plating, or by a plating method such as electroplating or electroless plating. The magnetic recording medium obtained by the foregoing method is termed a magnetic recording medium of the metal thin film type. The magnetic recording medium of this type is not limited to a pure metal thin film, but may have a metal nitride thin film formed by nitriding a metal thin film in the process of forming the metal thin film or a partially oxidized metal thin film formed by partially oxidize a metal thin film. Such magnetic recording media are generally termed thin-film type magnetic recording media. The magnetic recording medium of the thin film type is also sometimes called a magnetic recording medium of the non-binder type because no organic binder is employed.
Such a magnetic recording medium of the metal thin-film type is attracting attention because ferromagnetic metal having a large saturation magnetization can be formed into a thin film without the use of a binder. As a result, the coercive force can be made high, the film can be made thin in comparison with the coating-type magnetic recording medium, and, in addition, the manufacturing process can be simplified.
The metal thin film of such a metal thin-film type magnetic recording medium appears to the unaided eye to have a uniform and smooth metal surface. Microscopically, however, the metal thin film has a rough surface structure in which metal corpuscules are arranged side by side.
Further, magnetic elements such as Fe, Co and Ni are electrochemically active and do not have a sufficient anti-corrosion efficiency. Therefore, the magnetic recording medium of the metal thin film type suffers from a corrosion problem and is inferior in durability to the magnetic recording medium of the coating type. Particularly, in the case of a magnetic recording medium used in a cassette tape, a video tape, or the like, the surface of the medium is squeezed by a magnetic head during the recording/reproducing operation. If corrosion occurs on the metal thin film, the corroded material may be dislodged from the metal thin film by friction to thereby cause fouling of and possibly damage to the head, even if the amount of corrosion is extremely slight.
Further, as mentioned above, the magnetic recording medium of the metal thin-film type has a problem in that its durability is poor. That is, in a magnetic recording medium of this type large amounts of friction occur because the metal thin film has such a smooth surface that sticking is apt to occur. Therefore, magnetic recording medium of the metal thin film type is inferior in durability or the like during a still-viewing operation in a VTR to a magnetic recording medium of the coating type.
In order to improve the durability of the magnetic recording medium of the metal thin-film type, there have been proposed, for example, a method in which surface nitriding processing is performed by ion plating (see Japanese Unexamined Patent Publication No. 53-33806), a method in which a silicon nitride film is formed by sputtering (see Japanese Unexamined Patent Publication No. 53-30304), a method in which a magnetic film is exposed to a discharge in an atmosphere of nitrogen gas or the like to thereby form a nonmagnetic surface layer (see Japanese Unexamined Patent Publication No. 53-85403/1978), a method in which a metal nitride thin film is formed on a magnetic metal thin film (see Japanese Unexamined Patent Publication No. 54-143111).
For producing a magnetic recording medium of the thin-film type, there has been proposed a method in which iron vapor obtained by heat-evaporation is made incident on a substrate while the substrate is irradiated with ions of a mixed gas containing nitrogen using an ion gun so that a magnetic layer of iron nitride thin film is formed on the substrate (see Japanese Unexamined Patent Publication No. 60-231924).
Particularly, an iron nitride thin film in which at least iron and nitrogen are contained in a magnetic film is seen as promising for a magnetic thin film of the magnetic recording medium of the metal thin-film type because of its high corrosion resistance and its low cost as a raw material (see Japanese Unexamined Patent Publications Nos. 60-28028 and No. 60-236113).
For forming the foregoing iron nitride thin film, first, a method employing reactive ion plating has been proposed (see Japanese Unexamined Patent Publication No. 60-236113 and Japanese Journal of Applied Physics, pp. 1576-1579, vol. 23, 1984). In such a method employing ion plating, however, there are problems in that the degree of vacuum becomes low in forming a film, thereby lowering the quality of the film (for example, deterioration in the squareness ratio), and in that parameter control in forming the film is difficult. Alternatively, there has been proposed a technique in which nitrogen is ionized using an ion gun and a film is formed while the thus obtained ions are applied in the form of an ion flow to a film forming portion (see, for example, Japanese Unexamined Patent Publications Nos. 60-231924 and No. 61-926). According to this method, the energy, the quantity, etc., of the nitrogen ions can be controlled independently of the iron vapor flow to thereby make it possible to form a film of higher quality. The method using a nitrogen ion flow generated by an ion gun, however, has a disadvantage in that the rate of film formation is low, making it necessary to increase the caliber of the ion gun and increase the electric current for the power source of the ion gun so as to make the film formation rate sufficiently high. For example, according to the article "Collected Preliminary Papers for 1984 Autumn Meeting of Institutes of Metals", p. 470, an iron nitride thin film is formed under the condition that the nitrogen ion current density is 0.2 mA/cm.sup.2 and the iron evaporation rate is 20 .ANG./sec. This evaporation rate is considerably lower than the evaporation rate (500-2000 .ANG./sec) of a CoNi-O magnetic recording medium, which has been widely investigated. However, it is very difficult technically to improve the ion current density in the case of using an ion gun.
The conventional method of producing a magnetic recording medium of the iron nitride thin-film type and the conventional apparatus for practicing this method have the foregoing disadvantages. Accordingly, it has been required to develop a method for producing a high quality magnetic recording medium of the iron nitride thin film type with a high production efficiency, and to develop an apparatus for practicing such as method.